High voltage (HV) and medium voltage (MV) cables are used for power distribution on land and in the sea. Such cables often uses an extruded insulation system and comprise an electric conductor that is surrounded by an insulation system and a number of layers of different materials having different purposes and uses, e.g. as many as eight to nine layers. The insulation system comprises an inner semi-conducting layer closest to the conductor, an insulation layer externally of the conductor screen and an outer semi-conducting layer.
It is common to use the term cable core, and generally the cable core comprises the main layers of an inner electric conductor, and the insulation system as described above and comprising at least an inner semi-conducting layer, an insulation layer and an outer semi-conducting layer. The term cable core usually also includes, externally of the insulation and from inside out, any optional swelling material, if used, a metal sheath and an extruded oversheath that may be semi-conducting or insulating.
A pre-fabricated joint can be used when jointing two lengths of cable. The pre-fabricated joint comprises a pre-moulded/pre-fabricated joint body of e.g. rubber that is used to restore the insulation system when jointing the two lengths of cable. The conductors of the cable cores are jointed and the insulation systems of the jointed cable cores are restored in the joint body. This type of joint is commonly used for jointing high voltage cables with an extruded insulation system, normally comprising cross linked polyethylene (XLPE). For submarine cables, the pre-fabricated joint body is mounted in air at atmospheric pressure and then placed inside a water tight metal casing. The metal sheath of the cable core is normally connected to the casing through soldering, thereby achieving an overall watertight design for the joint.
For submarine DC cables containing one cable core, a rigid joint consists of one of these metallic casings containing a cable core joint, which casing normally is placed in an outer container that is also used to connect the armour layers of the cable. For submarine AC cables containing three cable cores, a rigid joint consists of three of these metallic casings, each containing a core joint, which casing are normally placed in an outer container which is also used to connect the armour layers of the cables. The entire joint including the outer container is commonly referred to as a rigid joint.
When such a rigid joint is used for jointing of submarine cables, the outer container that surrounds the water tight metal casing/casings has a mechanical function of protecting the casings and it is usually filled with water, when the cable and the rigid joint is submersed into the water. Thus, the inner water tight casing functions as a pressure vessel with an atmospheric pressure inside, and hydrostatic pressure of the water outside the casing. This results in a pressure gradient along the electrical core that is being jointed. The above described type of rigid joint with pre-fabricated rubber joint body has successfully been implemented for submarine cables at water depth up to approximately 600 m, corresponding to a hydrostatic pressure of approximately 6 MPa.
However, the question arises if such rigid joints could be used for large water depths, deeper than 600 m.
It has been found that for large water depths the scenario does not look well if a regular rigid joint is used. According to numerical analysis and experiments, an excessive deformation is expected over the extruded insulation in a critical transition region just outside the inner casing, where the cable core is entering into the inner casing. A significant reduction in the outer diameter of the cable core insulation occurs, so called necking, which is due to plastic deformation, yielding and/or creep of the cable insulation. Such deformations can significantly affect the optimal electrical characteristics of the extruded insulation, e.g. create undesirable consequences for the electrical field distribution over the cable and therefore cause its failure under operation.
The critical transition region, where the cable core is close to entering the inner casing of the joint, is severely affected by a significant pressure difference or gradient. Outside the casing, the cable core is exposed to a high hydrostatic pressure due to the large water depth, while inside the casing the cable core is under atmospheric pressure. During operation, the insulation system will be heated which reduces the mechanical strength of the extruded insulation, making the insulation even more susceptible to deformation.
Also, over this critical transition region, the high pressure difference creates a significant unbalanced compressive stress state on the cable along its axial direction. Thus, at the same time as the excessive necking occurs, there is a tendency for the extruded insulation to be displaced along the axial direction of the cable core towards the interior of the casing, where the pressure is lower. In addition to negatively affecting the electrical properties, this could also affect the water tightness of the casing at the location where the core enters the casing.